The present invention relates to a surface acoustic wave filter utilizing surface acoustic waves, in particular to a surface acoustic wave filter that has a two-port acoustic wave resonator utilizing longitudinal mode coupling and a surface acoustic wave resonator connected thereto and formed on the same piezoelectric substrate therewith.
RF SAW (Surface acoustic wave) filters employed in vehicle telephones and portable telephones have pass bands mainly in the frequency range of from several hundred MHz to several GHz. The frequency and the necessary frequency bandwidth depend on the mobile telephone system being employed. In general, the bandwidth is required to be several percent in terms of specific bandwidth.
In addition, a low insertion loss is required for the SAW filters employed in these mobile telephones or the like. Therefore, the following SAW filters have been dominantly used for these applications. These are ladder type filters in which SAW resonators are connected in ladder as disclosed in Japanese Patent Laid-open Application No. HEI 5-183380, longitudinal mode coupled resonant SAW filters in which a plurality of inter digital transducers (IDTs) having comb-shaped electrodes are put between reflectors as disclosed in Japanese Laid-open Application No. HEI 4-207615, and a combination of these two type filters.
Any one of the SAW filters mentioned above can realize filters of relatively small insertion loss. When these SAW filters are employed in RF filters of portable telephones, piezoelectric substrates possessing relatively large electromechanical coupling factor k2 such as 36xc2x0 Y-X LiTaO3, 64xc2x0 Y-X LiTaO3, 41xc2x0 Y-X LiTaO3, or like, have been widely used. The reason is that the possible pass bandwidth of these SAW filters largely depends on the electromechanical coupling factor k2 of piezoelectric substrates formed thereon.
These piezoelectric substrates, however, have the following disadvantages. The conversion loss from a surface acoustic wave to a bulk wave on a substrate of these piezoelectric substrates increases as the thickness of the conductive films forming IDTs or reflectors increases. For this reason, the SAW filters show a large insertion loss when the filters are formed with an increased film thickness. On the other hand, when the film thickness decreases, the electrical resistance of the electrodes forming IDTs on the piezoelectric substrate increases. Therefore, the filters show a large insertion loss also when the filters are formed with a decreased film thickness. Thus there is a range of its optimum values for the conductive film thickness h formed on one of these piezoelectric substrates. The range of the optimum values in terms of normalized film thickness (h/xcex) normalized by the wavelength xcex of the SAW that propagates on the piezoelectric substrate is approximately several percent (approximately 3 to 8 percent).
Normally, a SAW filter in which a two-port SAW resonator utilizing longitudinal mode coupling and another SAW resonator are connected, is formed by patterning a conductive thin film for instance Al thin film or slightly Si and Cu containing Al film or like on a piezoelectric substrate. Then the two SAW resonators are formed with the same film thickness.
The bandwidth of the frequency pass-band required for the mobile communication system is broad such that the width is approximately several percent in terms of specific bandwidth. Therefore, the two-port SAW resonator utilizing longitudinal mode coupling is formed on a piezoelectric substrate possessing high electromechanical coupling factor k2 chosen from the substrates described above with a normalized conductive film thickness in the prescribed range. In addition, the electrode finger width W divided by the disposed electrode finger pitch P (W/P, abbreviating as a duty hereinafter) is set at approximately 50 to 65 percent for the purpose of reducing insertion loss in many cases.
The SAW resonator connected to the two-port acoustic wave resonator has an effect of attenuating signals in the vicinity of the two-port SAW resonator pass band. Since the transmission band and the reception band are located in a very close vicinity in mobile communication systems, the SAW resonator connected to the two-port SAW resonator is largely used in order to have the filter characteristics of attenuating either one of the transmission band or the reception band.
The SAW resonator connected to the two-port SAW resonator utilizing longitudinal mode coupling, however, is formed with the same film thickness as that of the two-port SAW resonator in which broad bandwidth is required. Accordingly, the SAW resonator connected to the two-port SAW resonator is apt to have a relatively large frequency difference between the resonance frequency and the anti-resonance frequency thereof.
In addition, since the wiring of the two-port SAW resonator utilizing longitudinal mode coupling is relatively complicated in its structure, the wiring carries a certain amount of inductance component. Other inductance components are added by the package, the bonding wire and so on.
As these inductance components are loaded to the SAW resonator connected to the two-port SAW resonator, the substantial frequency difference between the resonance frequency and the anti-resonance frequency of the resonator further increases.
The SAW resonator connected to a series arm of the two-port SAW resonator can obtain steep attenuation characteristics at the anti-resonance frequency and pass characteristics at the neighborhood of the resonance frequency of the connected oscillator. When the SAW filter is required to have an attenuation band at a frequency range in the vicinity of the high frequency side of the pass band, the anti-resonance frequency of the series arm SAW resonator is needed to locate in a vicinity of a low frequency edge of the attenuation band.
In this case, there is no problem when the resonance frequency of the SAW resonator connected to a series arm of the two-port resonator is located approximately at the center of the pass band of the two-port SAW resonator utilizing longitudinal mode coupling. However, when the resonance frequency of the SAW resonator connected to the two-port SAW resonator shifts towards the lower frequency side of the pass band, the filter loss at the high frequency side of the pass band increases. As the result, the pass band characteristics of the SAW filter as a whole have pass band characteristics having unfavorably dull shoulder. That is to say, the SAW filter loss at the high frequency side of the pass band becomes large in the case when the filter is required to have an attenuation band at a frequency range in the vicinity of the high frequency side of the pass band.
The present invention is carried out to solve these problems.
The object of the present invention is to provide a SAW filter having low loss and excellent cut-off characteristics, in particular to provide a SAW filter for mobile communication applications excellent in attenuation characteristics in the vicinity of high frequency side of a pass band.
A SAW filter of the present invention comprises a SAW filter, comprising a piezoelectric substrate, a first SAW resonator disposed on the piezoelectric substrate so as to have a first duty, the first SAW resonator being a two-port SAW resonator utilizing longitudinal mode coupling and having IDTs each comprising a pair of comb-shaped electrodes, and a second SAW resonator on the piezoelectric substrate so as to have a second duty smaller than the first duty, the second SAW resonator being connected to a series arm of the first SAW resonator on the piezoelectric substrate and having an IDT comprising a pair of comb-shaped electrodes.
The first duty and the second duty can be adjusted so that reflectance per electrode finger constituting the IDT of the second SAW resonator becomes smaller than that of the first SAW resonator.
In addition, the first duty and the second duty can be adjusted so that the electromechanical coupling factor of the IDT of the second SAW resonator coupled with the piezoelectric substrate becomes smaller than that of the IDT of the first SAW resonator coupled with the piezoelectric substrate.
Further, the wiring connecting the first SAW resonator having three or more of IDTs and the second SAW resonator can be disposed so as to go around a pad which is connected to a ground side electrode of at least one IDT electrode pair of the first surface acoustic wave resonator. Then, an increase of the frequency difference between the resonance frequency and the anti-resonance frequency of the second SAW resonator can be avoided.
For the piezoelectric substrate, a 36xc2x0 Y-cut X-propagation LiTaO3 substrate can be used. Other piezoelectric substrates, for instance, such as 64xc2x0 Y-cut X-propagation LiTaO3, 41xc2x0 Y-cut X-propagation LiNbO3, 45xc2x0 X-cut Z-propagation Li2B4O7 or the like can be used.
In addition, when a 36xc2x0 Y-cut X-propagation LiTaO3 substrate or a piezoelectric substrate equivalent with this is used, the film thickness of electrodes constituting the first and second SAW resonators is preferable to be in the range of from approximately 6 percent to approximately 8 percent in terms of normalized film thickness (h/xcex), and the first duty is preferable to be in the range of from approximately 45 percent to approximately 60 percent and the second duty is in the range of approximately 45 percent or less. Thus, a SAW filter having excellent frequency characteristics can be obtained.
Namely, the SAW filter of the present invention comprises a two-port first SAW resonator utilizing longitudinal mode coupling disposed on a piezoelectric substrate and a second SAW resonator connected to a series arm of the first SAW resonator. Here, the duty (electrode finger width/electrode finger pitch) of the second SAW resonator is made smaller than the duty of the first SAW resonator.
By adopting this SAW filter constitution of the present invention, the reflectance per electrode finger of the second SAW resonator can be set smaller than that of the two-port first SAW resonator utilizing longitudinal mode coupling to which the first SAW resonator is connected. Accordingly, the frequency difference between the resonance frequency fr and the anti-resonance frequency far can be made small even when effects of inductance components are added to the characteristics of the second SAW resonator. As a result, the loss increase at the high frequency side of the SAW filter pass band can be reduced and an excellent attenuation characteristics in the vicinity of high frequency side of the pass band can be obtained. In addition, a high productivity of the SAW filters can be obtained since there is no need for changing the film thickness between the first SAW resonator and the second SAW resonator.
The present invention avoids an unfavorable effect due to inductance components added to the second SAW resonator by making the values of the electromechanical coupling factor k2 and the reflectance small. Practically, the duty of the second SAW resonator is made smaller than that of the first SAW resonator so as to make the reflectance per electrode finger of the second SAW resonator be smaller than that of the first SAW resonator. Moreover, a similar effect can be obtained, for instance, by making thinner the film thickness of the electrode constituting the second SAW resonator. In this case, each film thickness of the resonators is required to adjust individually for a plurality of SAW resonators disposed on the same piezoelectric substrate. Accordingly, fabrication process of SAW filters becomes sophisticated sacrificing their productivity. The SAW filter of the present invention having the same film thickness h for the first SAW resonator and second SAW resonator can be produced with a high productivity.